1. Field of the Invention
The present invention relates to an electronic power steering apparatus for a vehicle, and in particular to an electronic power steering apparatus for a vehicle which is capable of providing a support structure of a ball nut engaged with a motor shaft on a rack pinion type gear mechanism having a motor for thereby enhancing a steering force.
2. Description of the Background Art
Generally, a steering apparatus for a vehicle is directed to changing a running direction of a vehicle and includes an operating mechanism formed of a steering wheel, a steering shaft and a steering column, a gear mechanism for changing an operating direction of the operating mechanism, and a link mechanism for transferring a movement of the gear mechanism to a steering wheel.
The gear mechanism includes a worm sector type formed of a worm gear and a sector gear, and a rack pinion type which is operated based on a combination of a rack gear and a pinion gear. The electronic power steering apparatus further includes a motor and a decelerator capable of enhancing a steering force of the gear mechanism.
FIG. 1 is a cross sectional view illustrating a conventional rack pinion type electronic power steering apparatus. As shown therein, the conventional rack pinion type electronic power steering apparatus includes a torque sensor 10 for receiving a rotational force from the operating mechanism, a pinion housing 20 which is installed across the torque sensor 10 and has a rack bar 30 therein, a tie rod 40 formed at both ends of the rack bar 30, and a motor 50 installed at an intermediate portion of the rack bar 30.
The motor 50 includes a motor housing 51 having a certain diameter larger than that of the pinion housing 20, a stator 52 installed closely to an outer surface of the interior of the same, a rotor 53 rotatably installed in an inner side of the stator 52, a motor shaft 54 extended toward the inner side of the rotor 53, and a ball nut 55 provided at one end in the interior of the motor shaft 54.
At this time, a screw groove is formed in an inner side of the ball nut 55 and an outer side of the rack bar 30, and a ball is disposed therebetween. Therefore, when the ball nut 55 is rotated, the rack bar 30 is moved in the axial direction.
One end of the motor shaft 54 is closely contacted with a step formed in the interior of the ball nut 55, and the other end of the same is fixed by a lock screw 56, and a stop ring 58 is installed in a rear side of the lock screw 56.
The lock screw 56 is capable of preventing an escape of the ball nut 55 and includes a threaded portion formed in an outer surface of the same for thereby being engaged with a threaded portion formed in the interior of the motor shaft 54. The stop ring 58 is capable of preventing an escape of the lock screw 56 and is inserted onto the outer portion of the groove 57 formed in the interior of the motor shaft 54, so that the inner side supports the lock screw 56.
In the conventional motor, the groove in which the stop ring is installed and the cross section of the stop ring are formed in a square shape, and the width of the groove is larger compared to the thickness of the stop ring for a smooth installation. Therefore, there is a certain operational gap therebetween.
Therefore, if the lock screw is loosened, since the lock screw is moved in the axial direction by the distance corresponding to the operational gap between the stop ring and the groove, the rotation and reciprocating movement of the ball nut fixed in the interior of the motor shaft becomes free.
Namely, since the rotational force transferred from the motor shaft to the ball nut is not properly obtained, a certain problem occurs in the steering force, and a noise occurs because the ball nuts collide with each other.
In addition, since a torque limit device is not provided, the motor may be damaged when an overload occurs.
Accordingly, it is an object of the present invention to provide an electronic power steering apparatus for a vehicle which overcomes the problems encountered in the conventional art.
It is another object of the present invention to provide an electronic power steering apparatus for a vehicle which is capable of improving the structures of a stop ring and a groove for thereby preventing a certain operational gap which is formed when installing the system and providing a torque limiting function when an over load occurs in the motor.
To achieve the above objects, there is provided an electronic power steering apparatus for a vehicle according to the present invention in which a groove is formed in a circular shape, an inner tapered surface is formed in an outer surface, a stop ring is formed to be expandable in the outer direction, and an outer tapered surface is formed in a portion opposite to the inner tapered surface at an outer surface for thereby being closely contacted with the groove based on a movement in the axial direction due to the expansion force of the stop ring wherein the electronic power steering apparatus for a vehicle includes a motor housing provided at an intermediate portion of a pinion housing having a rack bar therein, a ball nut inserted into an inner end portion of the motor shaft which is rotatable in the interior of the motor housing and moves the rack bar, a lock screw engaged to a portion of the motor shaft for thereby closely contacting the ball nut to the inner portion of the motor shaft, a groove formed in a rear portion of the lock screw, and a stop ring inserted into the groove and supporting the rear portion of the lock screw.